A seat of an automobile is typically fitted with a seat belt to restrain the vehicle occupant and prevent him or her from being thrown forward by the inertia force in a frontal vehicle crash or the like. A seat belt is typically provided with an emergency locking retractor (ELR) device that locks up a winding shaft for the seat belt when the vehicle comes to a sudden stop or crashes with another vehicle or a stationary object while permitting the movement of the vehicle occupant in normal situations. An ELR device locks up the winding shaft immediately upon detecting a deceleration level exceeding a prescribed level to prevent the seat belt from being paid out any further.
However, simply locking up the winding shaft would not prevent a certain length of the seat belt from being paid out as the part of the seat belt wound around the winding shaft wraps the winding shaft tighter even though the winding shaft is locked up and kept stationary. To make up for such a shortcoming of the ELR device, various forms of seat belt pretensioner devices have been proposed. A seat belt pretensioner device, for instance, rapidly moves a seat belt buckle in a direction to increase the tension of the seat belt. As disclosed in Japanese patent laid open publication No. 10-35412, Japanese utility model registration publication No. 3072446 and Japanese patent publication No. 2981393, it is known to actuate such a pretensioner device by using a power actuator including a cylinder, a piston received in the cylinder and a high pressure gas generator so that the buckle may be pulled, via a cable, by the piston which is in turn driven by the pressure of the gas generated from the gas generator.
Such a pretensioner device is required to be capable of a reliable activation because it can be effective only by removing a slack from the seat belt. In the structure where the cable is drawn into a cylinder, it is difficult to reliably seal the gap between the opening of the cylinder and cable. Even with the sealing arrangement disclosed in Japanese utility model registration publication No. 3072446, gas may leak from the gap between the cable and cylinder opening, and the gaps within the cable itself. To make up for such a leakage of gas, a correspondingly large amount of propellant is required, and this causes an increase in the size of the device. To ensure an adequate travel of the buckle, the piston is required to have a relatively large stroke, and this also increases the size of the device. Using a larger amount of propellant also means a larger amount of CO gas emission, and this is not desirable either.
Such a pretensioner device is typically placed in a small space between a seat slide rail and a center console, and this severely limits the allowable size of the pretensioner device.
It is conceivable to use a rotary arm, bell crank or the like to transmit power in a desired direction, but they do not provide so much freedom in layout as the cable. Also, vehicle crashes are not limited to frontal crashes but may occur at oblique angles. Therefore, the direction in which the vehicle occupant is thrown forward at the time of a crash may vary depending on the direction of the crash. Therefore, depending on the direction of the crash, the direction of the force applied by the vehicle occupant to the seat belt and buckle may not be predictable, and the power transmission mechanism could be subjected to a bending moment. For the device to be capable of withstanding such a force or moment, the device must be increased in both size and weight.